Download AP Test Review – energy

AP Physics – Little AP Test Review Helper - Energy
Conservation of energy and momentum are some of the most important concepts needed for success on the
AP Exam. In addition, the concepts of work and power are required to solve many questions on the AP
Test.
Work  the product of the component of a force along the direction of displacement and the magnitude of
the displacement.
W  F  r  F r cos
Power:
Rate that work is done.
PAvg 
W
t
also
P  F  v  F r cos
You can substitute energy for work in this equation. Power is the rate that something uses or
produces energy as well.
Given a power rating, you can find the energy involved or the work done by solving the power
equation for work.
A powerful device does work faster than a less powerful machine.
Energy  ability to do work.
Kinetic Energy  energy of motion
Potential Energy  stored energy
1
K  mv2
2
U g  mgh
F  kx
Hooke’s Law  Force exerted by a compressed spring
Potential Energy of a spring:
1
U S  kx 2
2
Electrical Potential Energy:
U E  qV 
Energy of a photon:
E  hf  pc
Potential Energy in Capacitor:
1
1
UC  QV  CV 2
2
2
Law of Conservation of Energy:
1 q1q2
4 0 r
Energy of an isolated system must remain constant.
This means energy before equals energy after.
Energy can be transferred from one form to another.
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Energy is lost when it is converted into a non useful form such as heat.
Work can equal:
(1) the change in kinetic energy
(2) the change in potential energy
Think of work as the energy that you add to a system. For example, you do 150 J of work to make
something move with 150 J of kinetic energy.
Work is equal to the area under a Force vs. Distance curve.
p  mv
Momentum: Think of it as inertia in motion.
Impulse: Ft  p Impulse is time taken to stop multiplied by the force needed to stop.
Law of Conservation of Momentum:
Momentum of an isolated system must remain constant.
This means that the energy before an event must equal the energy after the event.
Collisions: Two types, inelastic and elastic. You also have your explosions.
Inelastic collisions: Kinetic energy is not conserved - objects stick together.
Elastic collisions: Kinetic energy is (or almost is) conserved – objects bounce off one another.
Explosions:
Beginning momentum is zero - shooting a gun, astronaut throwing something away, etc.
Problem Solving Gouge: Typical types of problems usually require you to do one of the
following:

Convert some form of potential energy to kinetic energy so you can find the velocity of an object.

Use velocity of an object to find its kinetic energy so you can find the potential energy it began
with.

Find the potential energy of a charged particle in an electric field and then use its kinetic energy to
calculate its velocity, acceleration, or the force that is acting on it.

Use displacement of a spring to find the potential energy stored in the spring. Use this potential
energy to find kinetic energy when spring is released. Then you can find its velocity.

Use conservation of momentum to find the final velocity of an object. You might have to then find
the object’s energy.

Use conservation of energy to find an object’s velocity, then using this velocity, find a second
object’s velocity after a collision using conservation of momentum.
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
Use conservation of momentum to find an object’s velocity after a collision. Then use the velocities
to determine the change in kinetic energy for the system.
Many complicated problems will involve conservation of energy or momentum. The photoelectric effect
equation gives you the maximum kinetic energy of an electron. You can then find its velocity by using the
kinetic energy equation.
These problems might also involve a projectile motion component. A ball rolls down a slope, collides with
a second ball. It gains some sort of velocity and rolls across a table and then falls to the deck. You have to
find the horizontal distance it fell while it was falling.
The big picture:
The total energy before must equal the total energy after.
ETotal  U g  U S  K
1
1
 mgh  kx 2  mv 2
2
2
1
1
1
1
mgho  kxo2  mvo2  mgh  kx 2  mv 2
2
2
2
2
Useful shortcuts for simple problems used to find velocity of an object or using object’s velocity to find its
initial energy, &tc.
Gravity
Electric
Spring
Electrons
Power
1
mgh  mv 2
2
1
qV  mv 2
2
1 2 1 2
kx  mv
2
2
1
hf  mv2
2
A mass m undergoes a change in its height.
Charge q accelerated by charged plates with potential difference V.
Compressed spring
Energy of a photon transferred to an electron, or vice versa.
Involves work. You may have to go from energy to work to power then to voltage and
current P  IV
Energy and time Think power when you see energy and time, or Joules and seconds.
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